Gas separation and purification devices are used to selectively separate one or more target gasses from a mixture containing those and other gasses. One well known example is the use of certain membranes for the selective separation of hydrogen (H2) from a stream, flow, or region containing hydrogen in a mixture with other gasses. While the membranes for the selective separation of H2 might generally be polymers or metal, the polymer membranes are typically limited to use in low temperature environments. In circumstances where the membranes must be used in conjunction with high temperature processes, or processing, it becomes necessary to rely upon metal membranes.
In a typical example, the H2 may be the product of a reformation and/or water gas shift reaction of a hydrocarbon fuel, and the H2, following separation from other reformate or reaction gasses, may be used in a relatively pure form as a reducing fuel for the well-known electrochemical reaction in a fuel cell. The processes associated with the reformation and/or water gas shift reactions are at such elevated temperatures, as for example, reactor inlet temperatures of 700° C. and 400° C. respectively, that H2 separation, at or near those temperatures, requires the use of metal membranes. The metal perhaps best suiting these needs is palladium, which is selectively permeable to H2, relative to other gasses likely to be present, and has high durability at these operating temperatures.
Composite membranes of palladium (Pd) or its alloy, consisting of a thin palladium layer deposited on a porous metal (PM), oxidation resistant substrate, when integrated with the reformer or the water gas shift reactor, result in desirable H2 permeation flux and offer significant advantages towards system size and cost reduction. Pd—Ag and Pd—Cu-based alloys are required for extended membrane stability in a sulfur-free or sulfur containing reformate, respectively, with the former being quite important for fuel cell power plants requiring a number of start up and shut down cycles. For a palladium alloy membrane to be produced by electroless plating (EP) or certain other techniques, high temperature thermal treatment, e.g., in the 550° C.-650° C. temperature regime, in a controlled atmosphere is needed in the latter stages of the process. Because this thermal treatment will, or may, cause intermetallic diffusion of the porous metal substrate constituents into the Pd phase that is detrimental to H2 permeance, an intermetallic diffusion barrier, usually a ceramic, is typically placed on the surface of the porous metal substrate prior to the palladium plating. Examples of such techniques may be found in, for example, U.S. Pat. No. 6,152,987 and U.S. published applications US 2004/0237779 and 2004/0244590 by Y. H. Ma, et al. In the instances cited above, this ceramic interlayer is grown thermally, either as an oxide from the metal support or as a separate phase like nitride from N2 or NH3 decomposition or carbide from a carbon-containing gas stream. The palladium membrane support is thermally treated in air, ammonia, nitrogen or a carbon-containing gas at extreme temperatures and prolonged times to achieve this result. Additionally, following formation of the ceramic interlayer, the outer surface of that interlayer is activated by seeding it with the nuclei of the metal that is to form the membrane, i.e., Pd. This activation of that outer surface of the interlayer facilitates the subsequent electroless plating of the Pd onto that surface
Following the formation and activation of a ceramic layer on the metal support, the support is plated with Pd by being immersed in a Pd electroless plating bath. That electroless plating bath has typically been prepared from several components, including Pd and hydrazine (H2NNH2, or N2H4), combined at room temperature, i.e., 20-25° C. The ceramic coating on the support has been subsequently plated by immersion in the electroless plating bath for an interval at a temperature of about 60° C.
Pursuant to the foregoing process, the resultant palladium or palladium alloy, membrane is typically less than 50 microns (μm) in thickness, and has typically been formed on the exterior surface of a tubular chamber or conduit in which the reformation or water gas shift reaction occurs and/or the reformate flows. All else being equal, because an increase in the flux of hydrogen through the membrane can be obtained by reducing its thickness, there is incentive to reduce the thickness as much as reasonably possible while keeping mechanical strength. On the other hand, there are practical limits to the structural integrity and durability, and indeed even gas-type selectivity if the membrane becomes too thin. While the Pd membranes of the prior art may be fairly economical in their use of Pd, their location on the exterior of a tube or the like, subjects them to possible scratching or abuse during handling, which may result in leakage. In one example constructed in accordance with the prior art, the membrane had a thickness of about 18 microns and an H2 permeance of no more than about 12 m3/(m2 hr atm0.5) over an interval of more than 60 hours at 350° C.
The hydrogen flux through a membrane, J, can be described by Sievert's law, which is: J=Q/L (P10.5−P20.5]. The P1 and P2 are the partial pressures of hydrogen on either side of the membrane and the difference in the square roots of these pressures is the driving force through the membrane. The quantity Q/L in front of the driving force term is the permeance. Q is the permeability and L is the thickness of the membrane. The permeability, Q, is essentially the diffusivity of hydrogen through the membrane, D, times the partition coefficient or solubility, H, between the hydrogen in the gas phase and the hydrogen on the surface of the membrane. Thus, the permeability is: Q=D*H and the permeance is D*H/L.
To increase permeance, either the permeability has to be increased or the thickness reduced. Conversely, to increase physical durability and robustness, the thickness of a material with a given permeability is often increased, resulting in a decrease of the permeance of the resulting membrane.
Accordingly, there is a need to provide an improved Pd or Pd alloy membrane with a relatively high selective permeance to H2 flux. There is a further need to provide an improved Pd or Pd alloy membrane with a relatively high selective permeance and retained or improved durability and robustness.
Still further, it is desirable to provide such an improved Pd alloy in a configuration that minimizes exposure to physical abuse.